The invention relates to a method for friction stir welding to connect two components with a tool comprising a shoulder surface for subjecting the components to a process force in the region of a joining zone and a rotating stirring pin for stirring plasticized material or for plasticizing and stirring the components in the region of the joining zone, and to a device for connecting two components by friction stir welding with a tool that comprises a shoulder surface for subjecting the components to a process force in the region of a joining zone and a rotatable stirring pin for stirring plasticized material or for plasticizing and stirring the components in the region of the joining zone.
Friction stir welding makes it possible to integrally join two components that with the use of conventional melt welding methods cannot be welded, or that can only be welded with great difficulty. Friction stir welding is predominantly used for welding light metals. There is no need to add welding filler. In a known friction stir welding method, as described, for example, in WO 93/10935 A1, a stirring pin or welding pin, which extends from a shoulder surface of a rotating cylinder, is pressed into a butt joint of two components and is guided along said butt joint, wherein the shoulder surface of the cylinder subjects the components to a one-sided process force. As a result of the friction between the shoulder surface and the components heat arises, by means of which the material in the butt joint is plasticized, and the material is then stirred as a result of the immersion of the stirring pin. In the region of the butt joint or of the joining zone the components are supported on a counter-holder, which due to the process forces and contact with the stirring pin is subjected to very considerable wear and thus needs to be sporadically replaced correspondingly. In particular in the case of spherically-shaped large area components or spherically-extending joining zones, for example in the case of shell elements of an aircraft fuselage, wear of the counter-holders is considerably increased because apart from the process forces further factors such as differing thermal expansion coefficients and component tolerances have an additional negative influence on the state of the support. However, in particular the distance between the stirring pin head or the stirring pin tip and the counter-holder is a very critical process variable that must be closely observed in order to allow adequate stirring of the plasticized material. Even minimally insufficient immersion depth of the stirring pin, i.e. excessive spacing of the stirring pin tip from the support, results in a noticeable reduction in the quality of the weld seam. In addition to this, variations in the thickness of the components and different gap widths in the region of the joining zone result in a change in the immersion depth of the stirring pin. Furthermore, the stirring pin has a tendency, as a result of the process forces occurring, to drift in the transverse direction of the joining zone, which also results in a reduction in the quality of the weld seam.
A known solution for taking into account at least the variations in the thickness of the components in the region of the joining zone thus provides for pressing the shoulder surface in a force-controlled manner at a constant force against the components, and for designing the stirring pin so that it is movable along its vertical axis and is thus position controlled. To this effect, for example, in the direction of welding in front of the stirring pin a stylus device for sensing instances of unevenness in the material or variations in the thickness of the components is provided, which then immediately reports the aforesaid to an actuator for adjusting the immersion depth of the stirring pin. However, such a control device or regulating device is costly and complex, both from the point of view of device technology and control technology.
A known device-related and method-related solution, which at all times ensures adequate immersion depth or stirring, is also shown in WO 93/10935 A1 and in WO 00/02699 A1. The bobbin tool described therein comprises two opposite shoulder surfaces between which the components are clamped. One shoulder surface is formed by the cylinder, and the other shoulder surface by a plate-shaped head. The welding pin penetrates the cylinder and is rotatably held in said cylinder. In this method the components are subjected in the butt joint region by opposing process forces of equal force so that friction stir welding takes place so as to be quasi-neutral in terms of the vertical force. Apart from an always adequate immersion depth and good tracking of the stirring pin, this method provides other advantages, in particular a homogeneous heat input and little distortion of the components. However, the stirring pin is highly loaded because by way of it both the forces for rotating the plate and the forces for transversally moving the plate are inserted. Further disadvantages include a seam collapse on the sides of the plate surface (second shoulder surface) and comparatively expensive control technology or regulating technology. Moreover, welding thin-walled components is problematic because of the high heat input.
Very recent developments thus provide for weld tools with a stationary, non-rotating, shoulder and a rotating stirring pin. In this manner no heat is introduced into the component through the shoulder but only through the stirring pin.
In the case of curved component regions, U.S. Pat. No. 6,237,835 B1 provides for a slide that is movable in longitudinal direction of the seam, which slide is guided on the back of the components and is in active engagement with the stirring pin, for example having positive fit, so that the slide is quasi pulled towards the rear, and the component regions are clamped between the shoulder surface and the slide. However, with this solution it is not possible to react to varying material thicknesses during the welding process.